Add files to a bootable CD/DVD RW

Folks,

I need to do a BIOS update on my slightly ancient (2008 model) Dell desktop and the instructions suggested putting the update file on a bootable USB stick, booting that and running the update from there. I don't have a stick but I do have a couple of bootable disks and I'm wondering if I can add the file to one of them? Either that or build a new bootable disk that includes the BIOS update file?

Can anyone give me some pointers on how to do either of those tasks, please? I tried to look it up on the web and there are hundreds of articles about creating bootable disks, but I can't find any that talk about adding files to the standard ISO they all start with!

I'd really appreciate some help, thanks.

Also, should I back the system up before messing about with the BIOS? It hasn't been backed up for quite a while now.

First, if it works, don't mess with it.
Second, if you want to, or have to update BIOS, then do a complete backup first. Verify it as well. Better, do a drive image of your entire hard drive. Your choice: save time or save cost.
Third:
Depending on the PC and maker it is important: You may either use a CD/DVD or USB stick, or both. Consult your PC maker. If it is able to use a CD/DVD, your method (use a bootable disc) works. This way:
1. Use a FAT32 formatted USB stick. Copy the BIOS update file to it, as well as the update program file (execution or running file). [Or write them to a separate CD/DVD.]
2. Most BIOS update requires boot to DOS. Some maybe able to run inside Windows. If have option, select boot to DOS, rather than update from inside Windows... unless instructed otherwise. (That is, follow the instruction. Don't get too smart.)
3. Once booted, go to the USB stick, run the execution file in it as instructed. [Or swap in/out disc. Then run the execution program from the disc. You may encounter several disc swaps.]

If the BIOS update is data file *and* an execution file, you do not need to boot from USB. Boot from anything will allow you to access the files. To run it, most require boot to DOS (because it is a DOS file). So, check if the update is a DOS program file or not.
(I encounter some that are Linux programs. Now you need to boot to Linux to run it.)

Confusingly, some can run inside Windows and some can run outside Windows, in DOS mode. Some may require to boot to a different OS, or to pure DOS, not to Windows, then DOS Prompt.

Read the instructions carefully. If in doubt, **do not do it**.

Screw up BIOS and you may lose the entire PC/laptop. Fatal, no recovery.
Side note: Some update software also backup the original BIOS.